This invention relates to an apparatus for detecting specific color and, more particularly, to an apparatus which can reliably detect a specific color even if the background is dark.
A metered impression 12 that is printed on postal matter 10, as shown in FIG. 1, usually has three bar marks 14.sub.1, 14.sub.2 and 14.sub.3 at the right end. This pattern is red in color.
The pattern 12 is detected by a photoelectric converter section 16 in a metered impression pattern detector as shown in FIG. 2. Specifically, the postal matter 10 is illuminated by light from a light source 18, and reflected light is led through a lens 20, a slit 22 and a first half reflecting mirror 24 to a blue filter 26. Light transmitted through the first half reflecting mirror 24 is led through a second half reflecting mirror 28 to a green filter 30 and a red filter 32.
The blue filter 26 passes blue light, the green filter 30 passes green light, and the red filter 32 passes red light. These filters 26, 30 and 32 also, respectively, pass the blue light, green light and red light component of white light. The light beams from these filters 26, 30 and 32 are led to respective photoelectric converters 38, 36 and 34. The photoelectric converters 34, 36 and 38 convert the incident lights into corresponding electric signals.
The photoelectric converter section 16 comprises the light source 18, lens 20, slit 22, first and second half reflecting mirrors 24 and 28 and blue, green and red filters 26, 30 and 32.
When a metered impression 12 which is printed on a white background is detected by this apparatus, signals as shown in FIGS. 3(A), 3(B) and 3(C) are obtained from the photoelectric converters 34, 36 and 38 corresponding to the respective red, green and blue filters 32, 30 and 26. The color of the metered impression 12 can be detected through the comparison of these signals.
However, when a pattern 12 that is printed on a dark background such as a brown envelope is detected, signals as shown in FIGS. 4(A), 4(B) and 4(C) are obtained from the respective photoelectric converters 34, 36 and 38. These signals are difficult to compare with one another, so that it is difficult to accurately detect the color of the metered impression pattern 12.